how to parent a rigged grop of object using constraints to another object without messing up the constraint
close observation shows that your flaps control surface all are rigged and are parented to the box empty and it moves along with the rig and what you want is that the engine parts that you have made in another blend file that was imported to the current blend file needs to also follow along with the body . well parenting dosent hold good for these things since it just messes up everything be it the constraints etc .
you can use group instancing over here , heres how
so what you do is first select all the parts of the engine and press ctr g and the press shift a and select group instance you will see a replica of your engine parented to an empty axis use the empty to position the replica in the desired position . and with the  replica selected click on the main empty and parent it  to the empty using keep transform option . now you will see that along with the body the replica engine also moves when you move the empty . now you may notice that all the objects in the replica object look to be joined together to form a single object but you can edit every single object  it by editing the actual engine you can even scale the thrust controller empty and you will notice that even the replica will be able to do the trust vectoring , hope it worked , thanks and bye .